1. Field of the Invention
The present invention relates to a thermal flowmeter having heater resistors formed on a substrate for specific flow measurement.
2. Description of the Related Art
For example, as a flow sensor to detect the amount of air taken into an automobile internal combustion engine, it has become common to use a thermal air flow sensor capable of directly measuring the mass flow.
The recently proposed thermal flowmeters include those characterized in that sensor elements are formed on a semiconductor substrate of silicon (Si) or the like by micromachining technology. In such a semiconductor type thermal flowmeter, a heater resistor is fabricated on a several-micron film formed on the semiconductor substrate. Due to the thinness, its thermal capacity is small enough to allow high speed response and low power drive.
In addition, this type of thermal flowmeter makes it easier to form a fine structure for detecting the difference between temperatures measured upstream and downstream of the heater resistor and determining the flow direction to be forward or backward.
Examples of thermal flowmeters of this type are described in Japanese Patent No. 3342926 and JP-A-2006-200991.